The present invention relates to the technical field of solar batteries, and more specifically, to a method of fabricating a heterojunction battery.
The application of solar batteries has achieved remarkable progress in recent years. Crystalline silicon solar batteries, with advantages such as high photoelectric conversion efficiency and mature production techniques, have dominated the world's total solar battery output. Although the production process of crystal silicon solar batteries is advancing constantly, the lack of a good surface passivation mechanism and a range of emitter problems caused by the high temperature diffusion pn junction process in traditional production of crystal silicon solar batteries have not yet been improved, thereby limiting the enhancement of battery efficiency.
In the meanwhile, the industry always endeavors to explore a thin film solar battery manufacturing technique with low cost, high output and high efficiency. As the production process of hydrogenated amorphous silicon (α-Si:H) solar battery requires a low temperature (under 400°), without using silicon wafers and hence is convenient for large scale production, so it is highly valued and has been developed quickly. However, the photo-degradation of hydrogenated amorphous silicon solar batteries has not been well solved, and the photoelectric conversion efficiency is yet to be further improved.
Researchers have been devoted to combining advantages of crystal silicon batteries and thin film batteries to form higher-efficiency batteries. One approach is to use a wideband gap hydrogenated amorphous silicon layer as a window layer or emitter and use a narrowband gap monocrystal silicon or polycrystal silicon wafer as a substrate for forming the so-called heterojunction solar batteries. While taking advantages of the thin film production process, such batteries give full scope of performance and characteristics of crystal silicon and amorphous silicon materials and have development prospects of achieving high-efficiency while low-cost silicon solar batteries. It was reported in 1983 that Hamakawa et al. first adopted a-Si:H(p)/c-Si(n) heterostructured laminated solar batteries and achieved a photoelectric conversion efficiency of 12%. In 1991 Sanyo fabricated a-Si:H(p)/a-Si:H(i)/c-Si(n)-structured solar batteries with a conversion efficiency of more than 16% by using PECVD, and they referred to the structure of inserting between p-type a-Si:H and n-type x-Si a thin-layer intrinsic a-Si:H as a buffer layer as “HIT (Heterojunction with Intrinsic Thin-Layer) structure.” In 1994, their research made significant progress, i.e. fabricated on an area of 1 cm2 a HIT structured solar battery with a photoelectric conversion efficiency of 20.1%. Based thereon, Sanyo soon launched industralization research on HIT™ solar batteries and achieved industrial large scale production of HIT™ solar batteries. A HIT™ solar battery being produced with an area of more than 100 cm2 still has a photoelectric conversion efficiency of 17.3%, and output power of 96 pieces of battery component is 180 W, the battery components being named “HIT Power 21.” In 2003, Sanyo rewrote the highest record of conversion efficiency of HIT™ solar batteries with an area of 100 cm2 as 21.2%, and the industrial large scale production also hits a 18.5% photoelectric conversion efficiency.
With reference to FIGS. 1 and 2, the known basic fabricating procedure of a heterojunction battery is as follows: 1) first using a process similar to a crystal silicon battery to fabricate a textured structure at a surface of a wafer, so as to obtain light trapping effect; 2) using PECVD to deposit a 5 nm-10 nm-thick intrinsic a-Si:H and p-type a-Si:H layer on the front of an n-type CZ-Si wafer (180-250 um-thick) with a textured structure; 3) depositing a 20 nm-thick intrinsic a-Si:H and n-type a-Si:H layer on the back of the c-Si wafer; 4) using a sputter technique to deposit a TCO transparent conducting film on two sides of a battery, the conducting film also having ante-reflection effect; 5) plating a metallic aluminum on the back of the battery; 6) then making Ag electrode on the front of the battery by using a silkscreen printing technique. The entire fabricating procedure is implemented under 200□. The HIT battery having double-sided structure needs to plate an ITO transparent electrode on its back in step (4) and then make Ag electrode on both the front and back in step (6). Since the double-sided battery can receive reflected light from the ground, it can produce more electricity energy than a single-sided battery no matter whether the ground is smooth or not.
As is clear from the foregoing basic process, the traditional heterojunction battery further needs to fabricate a textured structure at a surface of a silicon wafer so as to obtain light trapping effect, such practice being basically the same as that of a crystal silicon battery. The process is very hard to control, which not only consumes a large amount of valuable chemicals but also consumes precious silicon wafer materials due to the corrosion reaction; in the meanwhile, since textures formed at the surface are quite uneven, and the uneven surface can hardly be completely covered by a thin intrinsic and p-type amorphous silicon film (a total thickness between 10 nm and 30 nm) during the procedure for forming heterojunction, which disadvantageously and seriously hinders the boost of open-circuit voltage. On the other hand, the traditional heterojunction battery uses sputtered ITO as a conductive oxide (TCO) material, whereas ITO materials are very expensive, which becomes the bottleneck of large scale production and utilization of solar battery. Heterojunction batteries, though having a high efficiency, is refrained from promotion by complex structure and process steps coupled with expensive materials. Therefore, there is current a need for a heterojunction battery fabricating method capable of reducing the production cost while enhancing the battery photoelectric conversion efficiency.